1. Field of the Invention
This invention relates to a hanging mercury drop electrodes, and more particularly, to a mercury drop electrode that is controlled by valving and employs capillary tubes that produce mercury drops that hang at the end of a capillary during electrochemical testing procedures, such as the trace analysis of inorganic cations and anions.
2. Description of the Related Art
The use of a hanging mercury drop electrode for the analysis of ions has long been used by electrochemists in the study of trace heavy metals analysis. When a chemical species reacts to the mercury electrode it happens by means of the application of an electrical voltage to the working electrode, which may be formed of mercury. When a ionic species reacts to the voltage, the ionic species reduces, or oxidizes, on the surface of the mercury electrode. This reactions causes a current to flow. The current, which is proportional to the amount of chemical species ions in the solution under test, is measured. Once the electrochemical test is completed the mercury drop at the end of the capillary is removed and a fresh drop of mercury is deposited at the end of the capillary for the next analysis.
Electrodes of this type are particularly useful in the field of electrochemistry as renewable working electrodes, particularly in the field of polarography, which is a form of voltammetry at a mercury electrode, rather than at a solid electrode such as gold. When this mercury electrode system is placed in a test solution, various analytes in solution which exhibit the ability to oxidize or reduce (lose or gain ) electrons can be measured. Voltage is applied to the working electrode which in turns generates a current. The current is generated at the time of the oxidation or reduction of the analyte or analytes to be studied in the electrolyte solution, and is proportional to the amount of analyte or analytes present in the test solution.
Once the mercury drop is present at the end of the capillary which is dipped in the solution to analyze, the magnitude of the current associated with the above oxidation or reduction can be recorded. In a typical arrangement, a waveform voltage is applied between the working electrode and a reference electrode, while current is measured between the working electrode and the counter electrode immersed in the solution under test. The solution under test must have an adequate amount of supporting electrolyte present in order for electrical conduction in through the solution to occur. A non-aqueous solvent system can be used with this system but a organic soluble salt must be used to accomplish this electrical conduction through the solution.
The two types of voltammetry which utilize the mercury hanging drop electrode are the mentioned polarography, and stripping analysis. In polarography, any waveform, such as linear sweep, normal pulse, differential pulse, square wave pulse, etc. is applied to the working electrode surface. In stripping analysis, a plating or deposition time is added to the experiment to preconcentrate the analyte of interest on the mercury drop. A reverse waveform is then applied to strip off of the drop the plated analyte of interest. Not all elements have the properties necessary for performing strip analysis, but the technique is useful particularly with respect to heavy metals.
There is a need for a mercury drop electrode arrangement wherein the mercury drop remains intact during the performance of the selected electrochemical test. Conventional mercury drop electrode arrangements do not have facility for precise control over the size of the mercury drop itself. The mercury drops, therefore, are of inconsistent size and often are made too large whereby they shear or fall off of the electrode during the electrochemical test.
With respect to replacement of the mercury drop, the prior art has used several forms of dispensing and dislodging systems for mercury drops. These systems include, inter alia, the use of large bore capillaries and gravity feed techniques, which have been used over the years, and the use of a U-tube type which allows the mercury drop to be formed in a vertical mode on the end of the U tube. Another known arrangement utilizes a dispenser which is responsive to a solenoid system which controls the size of the mercury drop. The plunger which forms a seal and is immersed in the mercury in the reservoir. This known arrangement is very cumbersome and oftentimes results in mercury being sprayed in the laboratory environment during the installation of the mercury.
In some of the more modern polarographic techniques, the formation of the mercury drop must be correlated with the acquisition of voltammetric data from the sample solution. Such correlation usually requires the implementation of an extensive timing function for each new drop of mercury. There is a need for a mercury drop electrode arrangement which can perform a complete voltammetric study using only a single drop of mercury.
The dislodge solenoid in some known mercury drop electrode systems is located at a predetermined location along a capillary length. Misadjustment of this "striking type" of dislodge system frequently leads to the breaking of capillaries, requiring replacement of the capillaries on a regular basis. There is a need for a mercury drop dislodge arrangement which does not damage the capillaries. There is additionally a need for a mercury drop electrode system wherein the mercury drop can be dislodged from a remote location.
Another problem which is present in the prior art relates to the entrapment of air during the valving of mercury. This can result in a discontinuity of the electrical path between the mercury drop and the point to which the electronic measuring and monitoring equipment is coupled to receive the voltammetric data. In addition, the inclusion of air in the mercury path can result in contamination of the capillary, since the removal of a mercury drop will permit contraction of the air pocket, resulting in withdrawal of the mercury into the capillary and the introduction of the sample solution into the capillary tip. There is, therefore, a need in the art for an arrangement which avoids such entrapment of air.
A still further problem in the prior art results from the fact that the contact to the mercury drop at the end of the capillary is achieved via the mercury reservoir. In such a commonly used system, the mercury reservoir acts as an antenna which causes interference in the current measurements made at the mercury drop itself.
It is, therefore, an object of this invention to provide a mercury drop electrode arrangement wherein the consumption of mercury, which is expensive and toxic, is reduced.
It is another object of this invention to provide a mercury drop electrode arrangement wherein a relatively large reservoir of mercury is not in electrical communication with the mercury drop.
It is also an object of this invention to provide a mercury drop electrode arrangement wherein the size of the mercury drop can precisely be controlled.
It is a further object of this invention to provide a mercury drop electrode system which does not require the implementation of a timing function for correlating the acquisition of voltammetric data from the sample solution
It is additionally an object of this invention to provide a mercury drop electrode system wherein an entire analysis of the solution under study is performed on that one drop.
It is yet a further object of this invention to provide a mercury drop electrode arrangement wherein the need to correlate multiple drops of mercury to voltammetric data for a given sample solution is not required.
It is also another object of this invention to provide a mercury drop electrode arrangement wherein the mercury drop can be dislodged from a remote location.
It is yet an additional object of this invention to provide a mercury drop electrode arrangement wherein the process of dislodgement of the mercury drop does not damage the capillary.
It is still another object of this invention to provide a mercury drop electrode arrangement wherein the likelihood of entrapment of air in the mercury reservoir is reduced.
It is a yet further object of this invention to provide a mercury drop electrode arrangement wherein electrical communication with the mercury drop does not include the mercury reservoir.
It is also a further object of this invention to provide a mercury drop electrode arrangement wherein interference effects caused by the mercury reservoir are eliminated.
It is additionally another object of this invention to provide a mercury drop electrode arrangement having an improved contact between the electrical connections of the mercury drop at the end of the capillary and a potentiostat which applies and measures the current.
A still further object of this invention is to provide a hanging mercury drop electrode arrangement which is easy to use and which achieves environmentally friendly goals, including reduction in the rate of consumption of mercury.
An additional object of this invention is to provide a mercury drop electrode arrangement which can be used with a variety of capillary materials, e.g., glass, Peek, Teflon.RTM., fused silica, etc.
Yet another object of this invention is to provide a mercury drop electrode arrangement which can be operated remotely.
Another object of this invention is to provide a mercury drop electrode arrangement which can deliver mercury drops irrespective of whether the arrangement is in a horizontal or vertical orientation.
A yet further object of this invention is to provide a mercury drop electrode arrangement wherein the characteristics of a sample solution can be tested irrespective of whether a hanging drop of mercury is present on the end of the capillary tube.
It is also an additional object of this invention to provide a mercury drop electrode arrangement wherein the capillary tube can readily be removed for replacement or cleaning.